In recent years, a technique for forming thin film transistors using a semiconductor thin film (with a thickness of several tens to several hundreds of nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors have been widely applied to electronic devices such as ICs or display devices. In particular, development as a switching element of a display device typified by a liquid crystal display device has been urgently developed.
In display devices such as liquid crystal display devices, amorphous silicon thin film transistors have been in practical use as switching elements. Channel-etched inversed staggered (or bottom-gate) TFTs including a channel formation region formed using amorphous silicon have been used in many cases.
A back-channel portion of an inversed staggered thin film transistor is exposed to a variety of work environment during the manufacture of the thin film transistor. For example, due to dry etching in a process for manufacturing the thin film transistor, plasma damage is caused or a fixed charge is generated in some cases. Further, moisture or an impurity adheres to or penetrates the back-channel portion after the manufacture of the thin film transistor in some cases. Change in qualities or contamination of the back-channel portion of the thin film transistor increases off current, leading to variation in characteristics and deterioration of the thin film transistor.
In order to reduce in off current of the thin film by removing an impurity of the back-channel portion, a technique in which the impurity adhered to the surface of back-channel portion is removed by surface treatment which i-type layer amorphous silicon in the back-channel portion between a source electrode and a drain electrode is subjected to plasma discharge under an atmosphere gas containing H2 or He has been disclosed (see Patent Document 1).